The objective of this study is to determine the effects of morphine on single neurons in the rat brain. Five areas (thalamus, posterior hypothalamus, amygdala, striatum and periaqueductal gray) will be studied. The thalamus is an area associated with pain pathways. Local application of a morphine antagonist in this area produces abstinence signs in morphine-dependent rats. This area is rich in high-affinity morphine binding sites. Direct microinjection of morphine into this area does not produce analgesia. The direct injection of morphine in the posterior hypothalamus, an area also rich in "opiate receptors", produces analgesia although this area is not classically associated with pain. The striatum is an area where the opiate receptor binding is the highest in the rat and in which direct injection of low doses of morphine produces analgesia. Periaqueductal gray and amygdala are the two areas richest in morphine binding sites; stimulation of periaqueductal gray can lead to analgesia; injection of morphine in this area in the rat leads to hyperalgesia. Finding that the amygdala is rich in the high-affinity binding of morphine is the first evidence that this area is a possible site of action of morphine. Using the above data it seems possible to make a precise study with the technique of microiontophoresis to determine the areas of the brain in which neuronal activity might be influenced by the direct administration of morphine and to test possible interactions with putative neurotransmitters. Morphine antagonists will be used to further define the neuronal response to morphine.